1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine and a printer, and more particularly, to an image forming apparatus having a function to charge the surface of a photoreceptor drum at an appropriate potential.
2. Description of the Prior Art
In an image forming apparatus such as an electrophotographic copying machine and a laser beam printer to form an image, the surface of a photoreceptor drum is charged by means of a main charger unit of Scorotron type and exposed to light to form an electrostatic latent image thereon. Since the quality of the image formed by this method depends on the charging condition of the surface of the photoreceptor drum, it is necessary that the photoreceptor drum be charged at an appropriate electric potential. A conventional method to promptly make the charging appropriate is such that the main charger unit is controlled by means of a voltage data obtained through a calculation by use of a function based on a grid voltage data and a main charging voltage data used for charging in the previous image formation. However, since the voltage data as it is obtained through calculation is used for the charging in image formation, an error may be caused and an appropriate surface potential is not always obtained.
For example, in a case such as the continuous copying where the present image formation is performed immediately after the previous image formation, a surface voltage which is substantially appropriate is obtained even though the photoreceptor drum surface is charged by using the voltage data obtained through calculation. However, when the present image formation is performed a considerable period of time after the previous image formation, it is impossible to obtain an appropriate surface voltage since the environmental conditions are different.